Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the reaction of a fuel being provided to an anode and an oxidant being provided to a cathode, thereby generating a potential between these electrodes. Such fuel cell assemblies are very useful and sought after due to their high efficiency, as compared to internal combustion fuel systems and the like. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products that are produced, such as water. In order to control the temperature within the fuel cell assembly, a coolant is provided to the fuel cell assembly. The coolant, typically water, is circulated throughout the fuel cell assembly via a configuration of coolant channels. The use of water within fuel cell assemblies makes them particularly sensitive to freezing temperatures.
Electrochemical fuel cell assemblies typically employ a hydrogen-rich gas as the fuel and oxygen as an oxidant where, as noted above, the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, having a catalyst layer formed thereon so as to promote the desired electrochemical reaction. The catalyzed membrane is disposed between two electrode substrates formed of porous, electrically conductive sheet material—typically carbon fiber paper. The ion exchange membrane is also known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION™.
In operation, hydrogen fuel permeates the porous electrode substrate material of the anode and reacts at the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous electrode substrate material and reacts with the hydrogen ions, and the electrons from the anode at the catalyst layer, to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers may be represented by the equations:Anode reaction: H2−>2H++2e−Cathode reaction: ½O2+2H++2e−−>H2O
Conventional PEM fuels cells have the membrane electrode assembly, comprised of the PEM and the electrode substrates, positioned between two gas-impermeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrodes, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
When flow channels are formed within these plates for the purposes of circulating either fuel or oxidant in the anode and cathode plates, they are referred to as fluid flow field plates. These plates may also function as water transfer plates in certain fuel cell configurations and usually contain integral coolant passages, thereby also serving as cooler plates in addition to their well known water management functions. When the fluid flow field plates simply overlay channels formed in the anode and cathode porous material, they are referred to as separator plates. Moreover, the fluid flow field plates may have formed therein reactant feed manifolds, which are utilized for supplying fuel to the anode flow channels or, alternatively, oxidant to the cathode flow channels. They may also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly assigned U.S. Pat. No. 3,994,748 issued to Kunz et al.
The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. Multiple electrically connected fuel cell assemblies, consisting of two or more anode plate/membrane/cathode plate combinations, may be referred to as a cell stack assembly. A cell stack assembly is typically electrically connected in series.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing a hydrogen-rich gas stream produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like. This process produces a hydrogen-rich gas stream efficiently as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer and related fuel processing apparatus well known in the art.
As discussed previously, the anode and cathode plates may be provided with coolant channels for the circulation of a water coolant, as well as the wicking and carrying away of water produced as a by-product of the fuel cell assembly operation. The water so collected and circulated through a fuel cell assembly in the coolant channels is susceptible to freezing below 32° F. (0° C.) and may therefore damage and impair the operation of the fuel cell assembly as the water expands when it freezes. It is therefore necessary to provide a method and apparatus, which may protect the fuel cell assembly during times of harsh environmental conditions.
U.S. Pat. No. 5,798,186 issued to Fletcher et al. on Aug. 25, 1998 discloses various electrical heating configurations for directly and indirectly thawing a fuel cell stack, which has frozen. Additionally, mention is made as to having compliant or compressible devices located within the stack manifold headers to accommodate the expansion of freezing water within the fuel cell stack. Such a system, localized only within the stack manifold headers, will not fully protect the entirety of the fuel cell stack or coolant channels from the effects of freezing and expanding coolant.
In particular, there are those situations where the start-up of the fuel cell assembly is desired after a time of inactivity in subfreezing environmental conditions. In such cases it has been discovered that attempting to circulate coolant through the coolant channels, in order to alleviate the freezing conditions within the fuel cell assembly, does not result in acceptable performance characteristics. When, for example, water is utilized as the coolant, the temperature of the fuel cell assembly typically causes localized freezing at the input to the small-dimensioned coolant channels, thereby partially blocking circulation therethrough and unduly lengthening the time required for start-up. When non-porous coolant channels or plates are utilized in conjunction with an antifreeze solution coolant, similar problems exist due the high viscosity of the antifreeze solution at low temperatures, again lengthening the time required for start-up.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a fuel cell assembly with a method and apparatus which overcomes the above-described drawbacks even in times of subfreezing temperatures.